In the past, and as illustrated in U.S. Pat. No. 6,323,814 by John T. Apostolos, entitled Wideband Meander Line Loaded Antenna, assigned to the assignee hereof, and incorporated herein by reference, wide bandwidth miniaturized antennas can be provided through the utilization of planner conductors which are fed through a so-called meander line which involves impedance changes to reduce the physical size of the antenna while at the same time permitting wideband operation.
The plates of the meander line loaded antennas are configured to exist above a ground plane and are spaced therefrom, with a meander line connecting a top plate or element to the ground plane. For operation in the 225 MHz to 2 GHz range, the height of the plates which are spaced from the ground plane can exceed five inches. Were the meander line loaded antennas operate down to 100 MHz, then the height above the ground plane would be on the order of ten inches;
For vehicle top applications when using an above-the-ground plane meander line loaded antenna, a ten-inch or more dome would have to be employed on the car top which is both unsightly and which can increase turbulent flow behind the antenna at vehicle speeds.
When these antennas are utilized on supersonic aircraft, anything having hard edges and existing above the skin of the fuselage results in intolerable turbulence which cuts down the efficiency of the aircraft.
in the past, for aircraft operation, a flush-mounted crossed slot antenna has been utilized in which slots depend down into a cavity some five inches. However in the application the overall size of the antenna is 30×30 inches. As a result, these yard square antennas require a significant amount of real estate on the skin of the aircraft, which real estate is in short supply.
There is therefore need to provide a small wideband flush mount antenna which does not affect aircraft aerodynamics while at the same time providing the required wideband performance.
Whether for a cell phone, PCS, 802.11 and/or GPS application such as that which is required for either hand held wireless communication devices or for use in vehicle mounted apparatus, or for use in either satellite communications from an aircraft or for VHF communications from the aircraft to the ground, what is required is an exceedingly small flush mount antenna which has a wideband frequency response.
Such a wideband frequency response is possible with the apparatus described in U.S. Pat. No. 6,323,814 and more particularly in co-pending patent application Ser. No. 10/123,787, filed Apr. 16, 2002 assigned to the assignee hereof the incorporated herein by reference. in this patent application the low frequency cut off of the meander line loaded antenna is decreased due to a cancellation of the reactance of the antenna by the reactance of the meander line and parasitic capacitance.
It was not at all obvious that a meander line loaded antenna in which the plates of the antenna existed above a ground plane could be submerged in a conductive cavity. It was also not immediately obvious that one could obtain the reactance cancellation obtainable in an above-the-ground plane meander line loaded antenna when using any kind of cavity.
Note, when others have attempted to flush mount antennas, the size of the cavities involved were such to preclude their use due to the massive size of the cavity involved.
Also, it was not clear that the gain of the antenna at the zenith and horizon would match the same characteristics as those of an above-the-ground plane meander line loaded antenna, especially when in a loop mode. It will be appreciated that having a horizon gain that approximates that of the gain at the zenith is quite important for omnidirectional general coverage for the antenna. For instance, if one is in a vehicle and one wants coverage at the horizon where cell sites are located, then it is important that the gain in the horizontal direction be such as to robustly communicate with the cell sites.
Moreover, if the antenna is utilized in a GPS mode, it will be appreciated that the horizontal dilution of position is much smaller when signals comes from satellites at or near the horizon, as opposed to satellites which are directly overhead. Thus, the gain of the antenna towards the horizon is indeed a critical factor and one which could not be predicted from a meander line antenna with a plate above its ground plane.
Thus, it is important for flush mount applications to be able to replace the crossed-slot flush mount antenna which is a yard by a yard in area with one with considerably reduced dimensions. This type of real estate savings is indeed important not only in aircraft but also in terrestrial vehicles where appearance is important.
Those skilled in the art will also appreciate that meander line loaded antennas such as described in U.S. Pat. Nos. 5,790,090; 6,313,716; 6,323,814; 6,373,440; 6,373,446; 6,480,158; 6,492,953; and 6,404,391 are known in which various techniques are utilized to create an ultrawide bandwidth for the antennas.
One antenna, called a cavity embedded meander line loaded antenna as described in U.S. patent application Ser. No. 10/251,131 filed by John T. Apostolos on Sep. 20, 2002 and incorporated herein by reference, involves a meander line loaded antenna flush mounted to the skin of an aircraft. It is a relatively wide bandwidth antenna, with a 3:1 ratio of high frequency cutoff to low frequency cutoff.
While such a 3:1 ratio is indeed quite useful in most applications, an even wider bandwidth would be appropriate for a number of applications. The problem associated with lowering the VSWR at least below 1800 MHz is that while the VSWR can be lowered significantly by placing a capacitor across the feed points to the meander line loaded antenna, it shorts out the antenna above 1800 MHz. Thus, a VSWR of less than 3:1 is possible for frequencies such as between 500 MHz and 1800 MHz.
However, since the capacitor acts to short the feed point above 1800 MHz, the use of a capacitor limits the potential upper band limit of such an antenna.
It will be appreciated that this type of cavity embedded meander line loaded antenna can be characterized as a loop type meander line loaded antenna in that a loop exists between the feed point across the top plate, down the cavity side, across the cavity bottom and up to the feed point. This loop path is like a coil and is responsible for inductive impedance which must be canceled if one is to have a low VSWR.
While the embedded cavity meander line loaded antenna can be characterized as a loop type antenna, so can the standard meander line loaded antennas in which the loop is formed from the feed point, across a top plate, across the meander line to an upstanding plate, through the ground plate and then up to the feed point. in fact, most standard meander line loaded antennas which are not embedded are of this type of configuration. These antennas are only broadbanded to the extent that the VSWR is relatively low across the entire band; and for that reason it is important to be able to cancel loop-induced inductive impedance at those frequencies at which inductive impedance is a factor.